This invention pertains generally to radar systems and, in particular, to a radar target identification apparatus.
There has always been a need for day/night, all-weather surveillance of designated areas to locate and identify objects therein such as aircraft, ships, satellites, and to distinguish between friend or foe. In meeting such a need, especially during periods of poor visibility, it is a common military technique to employ radar to detect moving targets. A skilled radar observer can classify moving targets according to speed, distance and bearing, however, the radar observer is unable to distinguish between moving targets as friendly or hostile since the radar echoes, or signature, from either friend or foe may be identical. Consequently, radar observation alone serves merely to alert the observer to the fact that a moving target is located at a certain point within the zone of observation.
It is therefore necessary that some form of the classic, challenge and reply be utilized to establish target identification. It is easy to furnish an aircraft or the like with a radio-receiver with which communication can be established between friendly forces when the craft enters the zone monitored by a friendly radar. The difficulty that is encountered is that it is more difficult for the craft to identify itself without disclosing its position or any other critical information to hostile forces. Furthermore, any equipment used as a means of identification between friendly elements must not compromise the security of either element. In order to achieve this end, the friendly element entering the field of a surveillance radar must be provided with some means of determining, without revealing the element's position, when penetration of the friendly radar field occurs. Further, since equipment issued to such friendly craft is often subject to capture by hostile forces, any identification means must be immune to compromise. Therefore, the employment of a fully-automatic identification friend or foe system is completely unacceptable since hostile forces using a captured automatic unit could penetrate a surveillence zone which is being surveyed by friendly forces. The system design must therefore provide some method whereby the authentic possessor of an identification, friend or foe (IFF) unit can transmit a unique identification.